The desire for increased functionality and performance of semiconductor integrated circuits and increased economy of manufacture has driven designs for such integrated circuits to extreme levels of integration density and reduction of size of electronic elements therein such as transistors. Complementary pairs of field effect transistors (FETs) of opposite conductivity types, referred to as NFETs and PFETs, have become the transistor technology of choice and ubiquitous in all but the most critical integrated circuit applications.
Since performance of a given transistor design may be compromised when the design is scaled to smaller sizes, many sophisticated transistor design features have been developed in order to preserve and enhance transistor performance of extremely small transistors. One such design feature applicable to field effect transistors is a gate structure using a high dielectric constant material (having a dielectric constant greater than eight but often greater than twenty) for the gate insulator and highly conductive metal for the gate electrode. Such a structure is often referred to by the abbreviation/acronym “HKMG”. Using such materials, the gate insulator can be made very thin and a highly conductive metal gate electrode can rapidly distribute electrical charge across the full surface of the gate adjacent the gate insulator; developing a much more nearly uniform electrical field for controlling current in the conduction channel of the transistor and improving the on/off resistance ratio which increases noise immunity and is particularly critical for low power operation required by power dissipation limitations incurred at high integration densities.
However, the metal chosen for the gate electrode has a profound effect on the work function of the gate and thus on the switching threshold of the FET. Since the same gate material must generally be used for both NFETS and PFETS transistors (to avoid severely complicating the manufacturing process and severely compromising manufacturing yield) and the switching thresholds of the respective NFETS and PFETS transistors of a complementary pair must be coordinated to avoid excessive currents in the series connected pair but has opposite effects on NFETs and PFETs and increases the magnitude of the switching threshold, the metal chosen for the gates cannot be ideal for either conductivity type. Therefore, it has been necessary to diffuse different materials into the high dielectric constant (Hi-K) material to adjust the work functions in opposite directions for the respective NFETS and PFETS by incorporating fixed charge materials into the Hi-K material. However, such processing is not entirely effective to achieve the desired switching thresholds for the NFETS and PFETS since the alteration in work function available therefrom is relatively small using materials which are capable of producing a stable shift of switching threshold when diffused into the Hi-K material and providing fixed charge materials at the interface of the metal gate and Hi-K material as has been thought necessary. Accordingly, the full performance potential of HKMG gate structures in FET designs has not been fully realized.